Autofocus systems typically rely on one or more sensors to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern SLR cameras use through-the-lens optical AF sensors, with a separate sensor array providing light metering, although the latter can be programmed to prioritize its metering to the same area as one or more of the AF sensors.
Through-the-lens optical autofocusing is now often speedier and more precise than can be achieved manually with an ordinary viewfinder, although more precise manual focus can be achieved with special accessories such as focusing magnifiers. Autofocus accuracy within ⅓,of the depth of field (DOF) at the widest aperture of the lens is not uncommon in professional AF SLR cameras.
Most multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection of the sensor using algorithms which attempt to discern the location of the subject. Some AF cameras are able to detect whether the subject is moving towards or away from the camera, including speed and acceleration data, and keep focus on the subject, which is a function used mainly in sports and other action photography. On Canon cameras, this is known as AI servo, while on Nikon cameras it is known as “Continuous Focus”.
The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of the optical system. A variation of autofocus is called an electronic rangefinder, a system in which focus data are provided to the operator, but adjustment of the optical system is still performed manually.
The speed of the AF system is highly dependent on the maximum aperture offered by the lens. F-stops of around f/2to f/2.8are generally considered optimal in terms of focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light.
Most consumer camera systems will only autofocus reliably with lenses that have a maximum aperture of at least f/5.6, while professional models can often cope with lenses that have a maximum aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters.